This invention relates to measuring the reactivity of individual nuclear fuel assemblies, and more particularly to determining whether the reactivity of an assembly is greater than a maximum permissible reactivity.
During the fuel cycle of a modern light-water reactor, spent fuel assemblies that have been removed from the reactor are typically placed in a fuel storage rack in a large water pool until certain highly radioactive but short-lived isotopes are exhausted. Although the short-lived isotopes may die out within a few years, the assemblies contain sufficient residual amounts of long-lived fissile and fertile isotopes such as U-238, PU-239, U-235 to pose a danger that the storage assemblies will "go critical" if minimum separation and orientation limits are not maintained in the rack. When existing nuclear power plants were constructed, it was believed that spent fuel would remain in such on site storage racks for three to six months. Accordingly, the racks were designed to accommodate, for example, fuel assemblies equivalent to one and one-third reactor cores.
In recent years, regulatory policies have made it virtually impossible to ship spent assemblies so the need has arisen to store more assemblies in the fuel storage pool than was originally planned. Increasing the number of assemblies stored in a given area decreases the spacing and increases the nuclear interaction between assemblies. Thus, there is an upper limit to the density of fuel assemblies that can safely be stored in a fuel storage pool of a given size. The regulatory authorities have established a maximum permitted effective reactivity of the rack in order to provide an adequate safety margin. This maximum limit, however, has not been increased in response to the need for storing more assemblies in a given storage area. Therefore, a way must be found to justify storing spent assemblies more closely together without exceeding the reactivity limit.